Optical apparatus using slitted masks for detecting rotary motion of object

ABSTRACT

A digital angle readout subsystem for an infrared horizon tracker provides a position reference for the tracker and a readout counter for indicating the angular displacement of the tracker from the reference position. The tracker is coupled to a movable mirror, which moves in synchronism with the tracker. The mirror reflects light projected through a primary mask and causes the light to strike a secondary mask at a location which varies with movement of the mirror. Photodetectors behind the secondary mask are responsive to the moving light patterns to provide electrical signal indications of the reference position and angular displacement of the mirror. The counting portion of the primary mask has evenly spaced slits and bars, the secondary counting mask includes left and right sections for splitting the field of movement of the tracker into two parts. Each section has four individual subsections shifted 90* in phase from an adjacent subsection. Switching between left and right counting sections is provided by switching pulses produced by primary and secondary reference masks. The reference masks have a pattern based on a vk lambda mathematical difference set and provide a reference position signal for the tracker.

t Unite States Patent Knight et a1.

1541 OPTICAL APPARATUS USING SLITTED MASKS FOR DETECTING ROTARY MOTIONOF OBJECT [72] Inventors: Sheldon Knight, Mountain View;

Kerwin Peter McCarron, Sunnyvale,

both of Calif.

[73] Assignee: Quantic Industries, Inc., San Carlos,

Calif.

Filed: is June 14,1968

211 Appl.No.: 737,274

[52] US. Cl. ..356/114, 250/225, 250/237, 356/150, 356/152 [51] Int. Cl...G01b 11/27, G0lb 9/10 [58] Field of Search ..356/150, 153, 169,138,152, 356/170, 114; 350/162; 250/237 2! '1 135x44 225E! ,j $1 l I I TF7Fi I 7%??9? 17 [451 Oct. 10,1972

726,352 1966 Canada ..356/ 169 Primary Examiner-Ronald L. WibertAssistant Examiner-J. Rothenberg Attorney-Flehr, Hohbach, Test,Albritton & Herbert ABSTRACT A digital angle readout subsystem for aninfrared horizon tracker provides a position reference for the trackerand a readout counter for indicating the angular displacement of thetracker from the reference position. The tracker is coupled to a movablemirror, which moves in synchronism with the tracker. The mirror reflectslight projected through a primary mask and causes the light to strike asecondary mask at a location which varies with movement of the mirror.Photodetectors behind the secondary mask are responsive to the movinglight patterns to provide electrical signal indications of the referenceposition and angular displacement of the mirror. The counting portion ofthe primary mask has evenly spaced slits and bars, the secondarycounting mask includes left and right sections for splitting the fieldof movement of the tracker into two parts. Each section has fourindividual subsections shifted 90 in phase from an adjacent subsection.Switching between left and right counting sections is provided byswitching pulses produced by primary and secondary reference masks. Thereference masks have a pattern based on a vklt mathematical differenceset and provide a reference v vnsis f t t ma er- 9 Claims, 7 DrawingFigures PATETEMBT 10 m2 SHEET 1 BF 4 a m W 4 M. Z

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Sheldon A. Kni BY Kerwin Peter McCarron PATENTED URI 10 I972 SHEEI 4 [IF4 OPTICAL APPTUS USING SLITTED MASKS FOR DETECTING ROTARY MOTION OFOBJECT The present invention is in general directed to optical apparatusand more specifically to apparatus for detecting the angulardisplacement of a movable object from a reference position of theobject.

In the field of space navigation and attitude control, the attitude ofthe spacecraft must be referenced relative to the earth. As disclosed incopending application, Ser. No. 474,613, now US. Pat. No. 3,495,085 inthe name of Sheldon A. Knight, entitled Radiation Gradient SensingApparatus and assigned to the present assignee, one method ofreferencing is to sense the gradient of infrared radiation between theearth and space. This device is termed an infrared horizon sensor wherethe sensor tracks the horizon. The tracking angle is thus a measure ofthe pitch or roll of the spacecraft. In order to measure this angle anangle readout subsystem is provided and is also disclosed in the abovecopending application.

It is a general object of the present invention to provide improvedoptical apparatus for sensing the angular displacement of a movableobject whose angular position is related to that of a horizon sensor.

It is another object of the invention to provide apparatus of the abovetype which is simpler and more reliable in operation and is capable ofcovering a relatively wide angular range.

In accordance with the above objects there is provided optical apparatusfor detecting the motion of movable objects which includes slittedprimary and secondary counting masks. Means are provided for projectinglight through the primary mask to strike the secondary mask at alocation thereon which varies with movement of the movable object. Theinvention more specifically, comprises a secondary counting mask havingslits therein equal in number per unit length to the primary mask slits.The secondary mask has at least two adjacent sections in a directionperpendicular to the axis of movement of the movable object. The twosections are spaced so that their slits have a 90 phase difference.First and second light sensitive means are behind the two sections.

The optical apparatus of the present invention also determines theangular displacement of the movable object from a reference position.Such apparatus includes primary and secondary reference masks havingtransparent slits and opaque bars for projecting light through the slitsin the primary mask to strike the secondary mask at a location thereonwhich varies with movement of the movable object. Light sensitive meansbehind the secondary mask detect the position of the movable objectwhich will cause the slit pattern of the first mask to strike the secondmask exactly on the bar pattern of the second mask. This is the angularreference position of the object. The invention comprises secondary andprimary reference masks having a predetermined bar and slit patterns,one being the reverse of the other, and the patterns being based on amathematical constant different set of the vkk type. The modulus, v, ofthe set is the number of units in the set which determines the totalnumber of slits and bars; k, is the number of members in the set whichdetermines the number of opaque spaces or bars in the pattern; and A isin mathematical terms the number of ways the differences form in theset. The ratio (A l v determines the light transmittance of a first andsecond mask for positions of the movable object other than the referenceposition.

Other objects of the invention will be more clearly apparent from thefollowing description.

REFERRING TO THE DRAWINGS:

FIG. 1 is a schematic view of an optical readout system embodying thepresent invention.

FIG. 2 is a plan view of the mask of the optical apparatus taken alongline 2-2 of FIG. 1;

FIG. 2A is an alternative embodiment of FIG. 2;

FIG. 3 is a greatly enlarged view of a representative reference maskwhich is useful in demonstrating the layout of the actual referencemask;

FIG. 4 is a block diagram showing suitable electronic circuitryassociated with the optical apparatus of the present invention;

FIG. 5 shows curves of the signal outputs of photodetectors employed inthe present invention which are useful in understanding the invention;and

FIG. 6 shows additional curves related to other photodetectors which areuseful in understanding the invention.

Referring first to FIG. 1, a tracking mirror 1 1 is illustrated which issynchronized for movement with an infrared tracker which is caused toalways point at the horizon of the earth. The specific mode of controlof this mirror is disclosed in the above pending application. Thesubsystem of the present invention relates the amount of movement andposition of the mirror to the spacecraft in which it is normallyinstalled. In other words, the present invention provides an improvedreference means for the readout mirror and an improved counter todetermine the angular displacement from this reference position. Mirror1 1 rotates about an axis 12 in either of two directions indicated andhas in front of it an objective lens 13 for focusing light from a source14. Light source 14 is located behind a mask assembly 16, shown in planview in FIG. 2, and projects light through slits in the mask towardmirror 11. Mask 16 includes a primary portion 16a through which light isprojected and a secondary portion 16b upon which the light strikes andbehind which are located several photodetector devices 17.

More specifically, and also referring to FIG. 2, the primary mask 16aincludes two primary reference masks 18 and 19 and a primary countingmask 21. Reference mask 18 includes a left portion 18a, a right portion18b, a transparent center portion 180, and terminating left and rightopaque end portions 18d and 182 respectively. Primary reference mask 19similarly includes left and right portions 19a and 1%, an opaque centerportion 190, and transparent left and right end portions 19d and 19e.

The secondary mask 16b includes two counting clusters 22 and 23 locatedat the left and right sides of the overall mask 16 respectively. Cluster22 includes four portions designated a, b, c, and d and similarlycluster 23 includes these four portions.

Secondary mask 1612 also includes secondary reference masks 26 and 27which are located one below the other. Located behind the varioussecondary masks are photodetector devices 17 the primed mask referencenumeral designating the photodetector as sociated with that particularmask section.

Referring now to the specific construction of the primary counting mask21 and secondary counting masks 22 and 23 the primary counting maskcomprises a series of transparent slits 30 and bars 31 which are equallyspaced and having a predetermined number per unit length. Secondarycounting masks 22 and 23 have the same number of bars and slits per unitlength except that each of the sections a through d are shifted 90 inphase from one another. Thus, in the case of sections a and c for bothmasks 22 and 23 the slit 30 of one mask will mate with the bar 31 of theother mask.

In operation light from source 14 passes through slits 30 of primarycounting mask 21 through the lens 13 and onto the mirror 11 where it isreflected back to the lens and imaged on either the secondary countingmask 22 or the secondary counting mask 23 depending on the mirrorposition. The effect of the superimposed masks will be to create avariation in light level as detected by photodetectors 23a through 23dand 22a through 22d between a dark level where a superimposed primarypattern of slits is in coincidence with the bars of the secondarypattern and a 50 percent light level where the patterns exactly mate.Thus, movement of mirror 11 will vary the light level between a percentand 50 percent level to create the photodetector waveforms as shown inFIG. 6. I

More specifically, the waveform designated a c is the combineddifferential signal output of photodetectors 23a23c' or 22a-22c. Thesame is true of the photodetectors bd. The a'c' curve is termed aninphase output and the b'd curve a quadrature output since it is shifted90 from the other curve. As is more fully explained in the KnightCopending Application, the two curves provide both counting informationand direction of movement either towards space or the earth.

The primary and secondary reference masks 18, 19 and 26, 27 areconstructed from a mathematical constant difference set of the vkh type.More specifically, v is the modulus of the set which is the number ofunits in it and which determines the total number of slits and opaquebars; k is the number of members in the set and determines the number ofopaque bars in the pattern; and is in mathematical terms the number ofways the differences form in the set. As illustrated, although the twopatterns of the primary and secondary reference masks are similar, thesecondary reference masks 26 and 27 are reversed as compared to theircorresponding primary. The ratio (A l)/(v) determines the lighttransmittance of the primary and secondary masks for all normalpositions other than the reference position where there is total lightextinction because of the reversal of the secondary masks.

The vklt type pattern is more specifically illustrated in FIG. 3 whichshows a representative pattern which has been simplified and has fewerbars and slits than does the specific pattern of the present invention.Mask 19a represents primary reference mask 19a with a transparent leftend 19d and opaque right connecting portion 19c. Mask 27 representssecondary reference mask 27 and 19"a the primary reference mask 19'a asit would be imaged on the secondary reference mask if the mirror 1 1(FIG. 1) had moved 2 units. The units are represented as zero through141 and represent the 15 units of the modulus which is in this specificexample a modulus of v equal to 15. Furthermore in this example, k isequal to 7 and )t is equal to 3 with a geometry of O, l, 2, 4, 5, 8, 10;this geometry indicates the units of the pattern which are opaque orbars. The others are transparent or slits. As illustrated secondaryreference mask 27' is the exact reverse of mask l9'a and in thisposition there would be a total extinction of light. However, byshifting the associated mirror so that the image of mask 19a has beenmoved 2 units as illustrated by 19"a, four units or slits of the masknow transmit light as indicated by the arrows. These four unitsrepresent the constant difference of the constant difference set andover a limited range will determine the transmittance level of thesuperimposed masks. Of course because of the transparent and opaque endand middle portions 19d and the transmittance will gradually rise to amaximum or decrease to a minimum. This is illustrated in FIG. 5 by thecurves 26" and 27" which represent the voltage outputs of photodetectors26 and 27 which are behind the secondary reference masks 26 and 27.

Referring now more specifically to FIG. 5 detectors 26 and 27 areserially connected in opposed phase so that, the resultant outputvoltage is represented by 26" minus 27". Moreover, referring to FIG. 2,since the pattern 18a is shifted one unit from 19a and the same is trueof 18b and 19b, the reference position where the imaged patterns providetotal extinction of light are offset one unit. This provides a doublepolarity pulse. The pulse associated with masks 18a and 19a is termed areference pulse and that associated with masks 18b and 19b representsthe normal position of the mirror to the optical axis of its lens. Thusthe reference mask provides both a reference pulse indicating the limitof travel of the mirror where counting should begin and a centerindication where the mirror is normal to the optical axis to provide forswitching between the secondary counting mask 22 and the secondarycounting mask 23.

An alternative embodiment of the reference mask portion of FIG. 2 isshown in FIG. 2A. The counting portions are identical to those of FIG.2. In the primary portion 16a of the mask a single reference mask 60replaces the two reference masks 18 and 19 of FIG. 2.

Similarly, in the secondary portion 16b a single secondary referencemask track 59 replaces masks 26 and 27. This provides a reduction in therequired field coverage of the readout lens and allows simplification ofthe illumination system.

More specifically, primary reference masks 68 and 69 correspond to masks18a and 19a (FIG. 2) and are placed in the same horizontal track by useof polarization coding. As indicated, mask 68 is horizontally polarizedand mask 69 vertically polarized. The two polarizations thus preventinterference between the reference patterns produced by the masks. Thesepatterns are received by correspondingly polarized secondary referencemasks 66 and 67 which have behind them photodetectors corresponding to26 and 27 (FIG. 4).

Primary masks 71 and 72 operate in the same manner. I

The 1 unit offset of FIG. 2 and its attendant function as explainedabove is provided in FIG. 2A by a smaller spacing 75 between masks 71and 72 as compared to spacing 78 between masks 68 and 69. The remainderof mask 60 is polarized as indicated.

Counting masks 22 and 23 are each associated with one-half the field ofmovement of the mirror 11 (FIG. 1). Thus when the mirror is normal tothe optical axis the associated photodetectors 23a'd' and 22a'd must becorrespondedly switched to provide a count for the proper field.Switching the photodetectors allows a lens with half the field coveragethat would be required if a single detector set were to be used. Thusthe lens required is smaller and simpler in construction than a lens ofgreater field coverage.

In addition the cornbination'of the a and 0 patterns provide for a totalextinction of light which provides for greater modulation and atherefore larger signal to noise ratio. Another advantage of thecounting mask configuration is that since the active detectors arelocated one above the other, problems with light gradients are lesssevere. If a shift in detector location is necessary the same sizeddetectors can be used. Furthermore, the image of the illuminated rulingprojected upon the detector ruling must shift the relative phases atleast +90 or a 27() to cause the system to stop counting. This allows agreat tolerance in the system before it does malfunction. And lastly thecounting pattern is easier to rule since it is uniform with regard toboth the primary and secondary patterns.

The general mathematical difference set theory used for constructing thereference masks is, from a theoretical standpoint, disclosed in anarticle entitled A Survey of Different Sets by Marshall Hall, Jr. in theproceedings of the American Mathematical Society, Volume 7, pages975-986, 1956. However, no practical application of the difference settheory is suggested in this article.

The device of the present invention was constructed and the followingset of parameters used:

The geometry of this pattern was 0, l, 2, 3, 4, 6, 7, 8, 9,12,13,14,16,l8,l9, 24, 26, 27, 28, 32, 33, 34, 35, 38, 41, 45, 48, 49,52, 54, 56.

The above parameters were chosen to provide a pattern where the barwidth was narrow enough to provide for a large angular resolution yetwide enough to match the resolution of the focusing lens. Also the 63units of the pattern allow it to be sufficiently large for the purposesof the preferred embodiment of the invention.

The operating circuitry of the angle readout subsystem is shown in FIG.4 along with the associated tracking circuit which is more fullydisclosed in the above mentioned copending application. Morespecifically, a detector array 31, sensitive to infrared radiation fromthe horizon of the earth, is coupled into a chopper 32, signal amplifier33, and demodulator 34. The output of demodulator 34 is fed to a nulldetect trigger 35 which indicates that the output of the demodulator hasreached zero. Because of the polarity arrangement of the detector array31, a null indicates that the detector is now sensing the point ofsymmetry of the horizon since the amount of infrared energy above thispoint is equal to the amount below. At this point the tracking mirror 11will be stopped in its rotation since it is locked on the horizon. Thisis initiated by the system logic 36 which is coupled to the mirrorthrough a gate 37, integrating amplifier 38, and up and down amplifiers39 and 40. The dashed line 41 represents the optical circuit from mirror11 to the detector array 31. Searching is initiated by the command inputalso coupled to system logic circuitry 36.

In order to provide a marker or reference pulse to indicate a knownposition of the mirror and also to feed this information to the systemlogic circuitry 36 and a readout counter register 42, referencephotodetectors 26' and 27' are coupled through a reference amplifier 43to a reference trigger 44 which in turn is coupled to the system logiccircuitry 36. Because of the opposed phase coupling of 26' and 27, asignal output 26"-27" (FIG. 5) having double polarity (positive andnegative) reference triggers and mirror normal triggers is produced.These triggers are coupled to summing and crossover networks 46 and 47.

Network 46 has as inputs inphase photodetectors 23a, c' and 22a, c.Network 47 has as inputs quadrature photodetectors 23b, d and 226', d.The mirror normal trigger provides switching between secondary countingphotodetectors 22' and 23 as the mirror passes from one half of itsfield into the other. Crossover network 46 is coupled to inphaseamplifier 48 and crossover network 47 is coupled to quadrature amplifier49. Amplifiers 48 and 49 provide inphase and quadrature triggers 51 and52 respectively to readout counter register 42 to provide a continuouscount up until the register is again reset by reference trigger 44. Inaddition as discussed more completely in the above mentioned copendingapplication interpolation outputs are also provided for higher digitalaccuracy.

In operation a search command input initiates searching for the earthshorizon. As mirror 13 which is associated with a horizon tracker crossesits reference position, counter 42 is reset. Counting continues untilthe tracker locks in on the horizon. The count stored in the counter isthus a measure of the angular displacement of the tracker and itsassociated mirror.

Thus the present invention provides improved optical apparatus in asubsystem for readout of the angular location of an infrared horizonsensor. Because of the higher modulation, greater accuracy in countingis achieved. Errors due to intensity gradients are reduced because ofthe closer packing of the detectors. Moreover a simpler lens design withadequate field coverage is achieved by the split fields. In additionmore reliable reference pulses are produced because of the constantdifference set format of the reference patterns.

We claim:

1. Optical apparatus for detecting motion of a rotatable object having areference reflecting surface which includes: slitted primary andsecondary counting masks and means for projecting light through saidprimary mask through a lens toward such surface to be reflectedtherefrom to strike said secondary mask at a location thereon whichvaries with movement of said rotatable object, the invention comprisingsaid secondary counting mask with slits therein equal in number per unitlength to said primary mask slits, all of said slits being directedparallel to the axis of rotation of said object, said rotatable objecthaving a predetermined field of movement, said secondary-mask having afirst cluster of four mutually adjacent slitted sections interceptingreflected light substantially only when said object is in one half ofsaid field, said sections being spaced so that the slits of borderingsections have a 90 phase difference, light sensitive means behind eachof said sections for providing an electrical signal indicative of therotation of said object in said one half of said field, said secondarycounting mask including a second cluster of four mutually adjacentslitted sections spaced similarly to said first cluster and interceptingreflected light substantially only when the object is in the other halfof said field, light sensitive means behind each of the sections of saidsecond cluster for providing an electrical signal indicative of therotation of said object in said other half of said field and switchingmeans responsive to the field half in which said object is located toselectively receive data from the light sensitive means associated withthat field half.

2. Optical apparatus as in claim 1 together with reference mask meansfor actuating said switching means said reference mask means includingprimary and secondary masks having a bar and slit pattern determined bya predetermined mathematical constant difference set.

3. Optical apparatus for determining the angular displacement of amovable object from a reference position which includes primary andsecondary reference masks having transparent slits and opaque bars meansfor projecting light through said slits of said primary mask to strikesaid secondary mask at a location thereon which varies with movement ofsaid movable object, light sensitive means behind said secondary maskfor detecting said reference position of said movable object which willcause the slit pattern of the primary mask to strike the secondary maskexactly on the bar pattern of the secondary mask, the inventioncomprising secondary and primary reference masks having predeterminedbar and slit patterns, the pattern of said secondary mask being thereverse of the pattern of said primary mask, said patterns being basedon a mathematical constant difference set of the .vkA type where v isthe modulus of the set which is the number of units in the set whichdetermines the total number of slits and bars, ,k is the number ofmembers in the set which determines the number of bars in the patternand A is in mathematical terms the number of ways the differences formin the set and the ratio (A l (v) determines the light transmittance ofsaid first and second masks for positions of said movable object otherthan said reference position.

4. Optical apparatus as in claim 3 in which v is equal to 63, kis equalto 31, and l is equal to 15.

5. Optical apparatus as in claim 4 where the geometry of said pattern is0, l, 2, 3, 4, 6, 7, 8, 9, 12,

6. Optical apparatus as in claim 3 in which said primary mask includesfirst and second adjacent sections one of said sections being offsetfrom the other by one unit.

7. Optical apparatus as in claim 6 where said secondary mask includesfirst and second sections corresponding to said sections of said primarymask.

8. Optical apparatus as in claim 7 where the light sensitive meansbehind said secondary mask provides a double polarity reference pulse atsaid reference position the direction of said polarity change indicatingthe direction of movement of said object.

9. Optical apparatus as in claim 3 in which said primary mask lieswholly within a single track and includes means for polarization codingsaid mask.

1. Optical apparatus for detecting motion of a rotatable object having areference reflecting surface which includes: slitted primary andsecondary counting masks and means for projecting light through saidprimary mask through a lens toward such surface to be reflectedtherefrom to strike said secondary mask at a location thereon whichvaries with movement of said rotatable object, the invention comprisingsaid secondary counting mask with slits therein equal in number per unitlength to said primary mask slits, all of said slits being directedparallel to the axis of rotation of said object, said rotatable objecthaving a predetermined field of movement, said secondary mask having afirst cluster of four mutually adjacent slitted sections interceptingreflected light substantially only when said object is in one half ofsaid field, said sections being spaced so that the slits of borderingsections have a 90* phase difference, light sensitive means behind eachof said sections for providing an electrical signal indicative of therotation of said object in said one half of said field, said secondarycounting mask including a second cluster of four mutually adjacentslitted sections spaced similarly to said first cluster and interceptingreflected light substantially only when the object is in the other halfof said field, light sensitive means behind each of the sections of saidsecond cluster for providing an electrical signal indicative of therotation of said object in said other half of said field and switchingmeans responsive to the fIeld half in which said object is located toselectively receive data from the light sensitive means associated withthat field half.
 2. Optical apparatus as in claim 1 together withreference mask means for actuating said switching means said referencemask means including primary and secondary masks having a bar and slitpattern determined by a predetermined mathematical constant differenceset.
 3. Optical apparatus for determining the angular displacement of amovable object from a reference position which includes primary andsecondary reference masks having transparent slits and opaque bars meansfor projecting light through said slits of said primary mask to strikesaid secondary mask at a location thereon which varies with movement ofsaid movable object, light sensitive means behind said secondary maskfor detecting said reference position of said movable object which willcause the slit pattern of the primary mask to strike the secondary maskexactly on the bar pattern of the secondary mask, the inventioncomprising secondary and primary reference masks having predeterminedbar and slit patterns, the pattern of said secondary mask being thereverse of the pattern of said primary mask, said patterns being basedon a mathematical constant difference set of the vk lambda type where vis the modulus of the set which is the number of units in the set whichdetermines the total number of slits and bars, k is the number ofmembers in the set which determines the number of bars in the patternand lambda is in mathematical terms the number of ways the differencesform in the set and the ratio ( lambda + 1)/(v) determines the lighttransmittance of said first and second masks for positions of saidmovable object other than said reference position.
 4. Optical apparatusas in claim 3 in which v is equal to 63, k is equal to 31, and lambda isequal to
 15. 5. Optical apparatus as in claim 4 where the geometry ofsaid pattern is 0, 1, 2, 3, 4, 6, 7, 8, 9, 12, 13, 14, 16, 18, 19, 24,26, 27, 28, 32, 33, 34, 35, 38, 41, 45, 48, 49, 52,
 56. 6. Opticalapparatus as in claim 3 in which said primary mask includes first andsecond adjacent sections one of said sections being offset from theother by one unit.
 7. Optical apparatus as in claim 6 where saidsecondary mask includes first and second sections corresponding to saidsections of said primary mask.
 8. Optical apparatus as in claim 7 wherethe light sensitive means behind said secondary mask provides a doublepolarity reference pulse at said reference position the direction ofsaid polarity change indicating the direction of movement of saidobject.
 9. Optical apparatus as in claim 3 in which said primary masklies wholly within a single track and includes means for polarizationcoding said mask.